The process of the invention concerns purification of dimethyl ether being produced from carbon oxides and hydrogen containing synthesis gas.
The conversion of synthesis gas to dimethyl ether is carried out in one or more reactors, in which synthesis gas is catalytically converted to methanol shown in equation (1), and dimethyl ether as shown in equation (2). The shift reaction also takes place and is shown in equation (3).CO+2H2→CH3OH  (1)2CH3OH→CH3OCH3+H2O  (2)CO+H2O→CO2+H2  (3)
Maximum conversion of synthesis gas is obtained when dimethyl ether is prepared at a stoichiometric ratio between hydrogen and carbon monoxide equal to one. At ratios above or below one less dimethyl ether is prepared. At maximum conversion (H2/CO≈1) the overall reaction takes place essentially according to equation (4):3H2+3CO→CH3OCH3+CO2  (4)
Carbon dioxide is soluble in dimethyl ether and in order to obtain the dimethyl ether product with a required purity, it is necessary to remove the carbon dioxide formed. Additionally, when carbon dioxide is removed the composition of the unconverted synthesis gas, which is recycled to the dimethyl ether synthesis reactor, is close to that of the make up synthesis gas used to prepare dimethyl ether, which is an additional advantage. Removal of carbon dioxide from the dimethyl ether product downstream the synthesis reactor can become very costly.
Three basic processes for disposing off carbon dioxide are known. In the first process dimethyl ether is synthesized according to reactions (1) to (3) above. A mixed effluent stream comprising unreacted synthesis gas together with any carbon dioxide present is then separated from the dimethyl ether product which also contains some unreacted methanol. The separated synthesis gas and carbon dioxide stream are recycled to the synthesis gas process stream entering the reactor. This process may conveniently be applied in a hydrogen rich synthesis gas having for instance a ratio between hydrogen and carbon monoxide above 5.
In the second known process a mixed effluent stream comprising unreacted synthesis gas together with carbon dioxide is separated from the dimethyl ether product. However, carbon dioxide is then subsequently separated from the synthesis gas. This can be done by washing this stream with for instance a suitable amine compound such as methyl di-ethanol amine, MDEA. The synthesis gas stream, which is free of carbon dioxide, is then recycled to the synthesis gas process stream entering the reactor. The carbon dioxide obtained may be employed in other processes for instance in the preparation of synthesis gas from natural gas by auto-thermal carbon dioxide reforming.
In the third known process only synthesis gas is separated from the dimethyl ether product and carbon dioxide. The dimethyl ether product thus contains both methanol and carbon dioxide. The separated synthesis gas is recycled to the synthesis gas process stream entering the reactor.
Various solvents are known in the prior art for removing carbon dioxide from mixtures with synthesis gas. The choice of solvent is dependent on the ability to dissolve dimethyl ether and carbon dioxide and the ideal solvent should have a high solubility for carbon dioxide and a low volatility.
U.S. Pat. No. 5,908,963 discloses a process for the preparation of dimethyl ether from synthesis gas, in which synthesis gas is separated from dimethyl ether product, and recycled to the synthesis gas process stream entering the dimethyl ether synthesis loop. The presence of excess methanol in the dimethyl ether product is the focus of the disclosed process and the removal of carbon dioxide is not addressed.
U.S. Pat. No. 6,458,856 discloses a one-step catalytic conversion process for dimethyl ether preparation. After catalytic conversion of synthesis gas to dimethyl ether the effluent from the reactor is separated into a vapour mixture comprising dimethyl ether, carbon dioxide and unconverted synthesis gas. The vapour mixture is scrubbed using a scrubbing solvent to remove dimethyl ether and carbon dioxide from unconverted synthesis gas. The scrubbing solvent comprises a mixture of dimethyl ether and methanol. The unconverted synthesis gas is recycled to the dimethyl reactor.
This reference also discloses prior art in which scrubbing solvents such as methanol, water, methanol/water mixtures, dimethyl ether or ethanol are used.
Dimethyl ether is a good solvent for carbon dioxide, but is very volatile, whereas methanol is a poorer solvent for carbon dioxide than dimethyl ether but has the advantage of being less volatile. A process for preparing dimethyl ether from synthesis gas which makes use of a solvent having high solubility for carbon dioxide and simultaneously low volatility is therefore desirable.